The international patent application WO 2011/159171 describes a passive RFID transponder with an UHF interface comprising an UHF antenna and a dual frequency LF-HF Interface comprising a LF antenna and an HF antenna. The dual frequency LF-HF Interface carries out an IP-X protocol in which a LF demodulator and an HF modulator is used. It is to be noted that the dual frequency LF-HF interface is intended to communicate with a same dual frequency reader. The passive RFID transponder further comprises an UHF rectifier and a LF rectifier which can both generate a power supply for this transponder. More particularly, a decision circuit is provided for selecting either the LF rectifier or the UHF rectifier as power generator depending on which one delivers the largest signal. The identification device needs from its power generator at least a first determined power level for the execution of the UHF communication protocol and at least a second determined power level for the execution of the IP-X communication protocol.
The passive RFID transponder further comprises a digital controller formed by an UHF logic unit, an IP-X logic unit and a single shared non-volatile memory (EEPROM). The transponder is arranged for detecting which kind(s) of reader is/are present and for responding accordingly. Depending on which energy source is present and the strength of that energy source, the digital controller selects either the UHF logic unit or the IP-X logic unit or both, and executes respectively an UHF protocol, an IP-X protocol or both.
The RFID transponder comprises a detector circuit forming an UHF detector and a LF detector which are respectively able to detect the presence of an UHF energizing signal and of a LF energizing signal before starting with one or two communication protocol(s). The document WO 2011/159171 only considers different situations wherein the transponder enters the field of a first UHF reader or the field of a second DF reader (DF being used for Dual Frequency, in this case LF-HF), as well as when the transponder enters simultaneously in both fields of the first and second readers. In this last case where both an UHF and a DF readers are present, the transponder may power up from either the UHF or the LF field or both. The method of operating the transponder comprises the step of causing the transponder, while being exposed to either an UHF or a LF energizing field or both, automatically to power up from the received energizing field(s) and to respond at either the first reader or the second reader or both of these. Thus, this document only considers the case where the transponder is in a power-down state and then enters a space wherein one or the other field or both fields is/are present, what causes the transponder to power up. As it is usual in passive transponders, a POR function (Power_ON Reset) is implemented in order to detect that a change between a power-down state and a power-up state (also named power_ON state) of this transponder has occurred, and to reset this transponder before starting the execution of one or the other protocol or both.
The document WO 2011/159171 ignores e.g. situations wherein one of the field disappears and not the other one. This document does not consider some problems related to the fact that two readers can at the same time provide respective energizing fields to the identification device while communicating at least with one of them, so that the transponder can remains in a power-up/power_ON state while an interrogation field intervening in the execution of one communication protocol is suddenly no more received by the transponder.